1. Field of the Invention
The present invention relates to a semiconductor fabricating apparatus. More particularly, the present invention relates to a polishing apparatus.
2. Description of the Related Art
Due to the increasing number of semiconductor devices incorporated in integrated circuits, multiple layers, such as multi-level metal interconnections, are utilized in most integrated circuit designs. In order to fabricating interconnects precisely, it is necessary for a wafer to have a good surface planarity.
Chemical-mechanical polishing (CMP) is widely used in a planarization process. In a chemical-mechanical polishing process, a rotation rate of a polishing pad, a rotation rate of a polishing head, a material of the polishing pad, and a pressure applied between the polishing pad and a wafer are all important factors that affect polishing results. Thus, the above-described factors need to be controlled during the chemical-mechanical polishing process. The polishing head is an especially important factor with respect to uniformity of the wafer. At present, the best polishing head is a floating polishing head, and it is widely used in the industry.
A conventional floating polishing head is simply described as follows. The floating polishing head comprises an air bag. The air bag has a tubular ring incorporated therein. The air in the tubular ring can be vacuumed, such that a wafer can be drawn up by the air bag.
In the conventional floating polishing head, there is only one ring utilized in the air bag. Normally, the tubular ring is kept at the same pressure as the air bag. There is no pressure difference between different areas of the wafer. The local pressures applied on different areas of the wafer thus are hard to control. This, in turn, causes the uniformity of chemical-mechanical polishing process are hard to control. The poor uniformity further causes a difference in thickness between the central area of the wafer and the edge area of the wafer. So, in practice, good wafer quality is difficult to obtain.
In addition, the difference in thickness between the central area of the wafer and the edge area of the wafer cause different lights to be reflected from the wafer. Different reflection lights with different colors easily lead to detection failure in the subsequent process, such as a defect scan process, performed on the wafer.